1. Field of the Invention
This invention relates to an improved non-flammable hydraulic fluid. It further relates to a hydraulic fluid of the type mentioned having improved shear stability and superior lubricating or wear preventing qualities.
2. Description of the Prior Art
In the prior art, numerous hydraulic fluids have been proposed. Some of these are of the mineral oil type which are advantageous in their good lubricating and anti-wear properties, but which are rather highly flammable and thereby unsuitable for certain uses. For example, in factories, such as iron works, in which machinery is often operated at high temperature, the hydraulic fluids used to control the machinery have frequently been a source of fire and danger. For this reason, there is a growing demand for non-flammable hydraulic fluids, and mineral oil type fluids are gradually being converted to non-flammable types.
Conventional non-flammable hydraulic fluids are mainly classified into three groups--phosphate esters; w/o (water in oil) emulsions; and water-glycol based fluids. The phosphate esters have good anti-wear qualities, but have a high cost and have the further disadvantage in that it is difficult to treat waste fluids derived from their use. While the w/o emulsions are relatively inexpensive, they tend to separate into their constituent components during use, and they also tend to suffer deterioration of some of their properties due to the propagation of bacteria. Moreover, they have poor wear reducing properties.
Water-glycol fluids commonly have high non-flammability, good stability and a relatively low cost. The water-glycol fluids, however, have poor anti-wear characteristics. Moreover, the fluids are deteriorated by metal dust resulting from metal wear thereby causing serious difficulties. For example, when conventional water glycol hydraulic fluids are used in hydraulic devices, e.g. vane pumps, designed and manufactured for use with mineral oil hydraulic fluids, the result is significant wear of the cam ring [which is made of ball-bearing steel (relatively soft steel)] even under mild conditions (i.e., a fluid temperature of 50.degree. C. and 70 Kg/cm.sup.2 or less of pressure). In extreme cases, the ring is worn in an amount in excess of 1,000 mg. While wearing of the vanes [which are made of high speed steel (harder than ball-bearing steel)] is relatively minor, the fluids tend to form deposits at the head of the vanes. Furthermore, metal dust or sludge resulting from wear tend to deposit on the filter, thereby decreasing its capacity. Some metal will also disperse into the fluids themselves making them turbid. Such metal dust or sludge catalyzes the oxidation of the fluids whereby the resultant oxidized fluid is characterized by poorer wear-preventing qualities.
In the past, there have been various attempts to improve the poor lubricating properties or poor wear-prevention properties of these fluids. For example, one method modifies the polyalkylene polyol which is often added to conventional water-glycol fluids. Another technique incorporates into these fluids such conventional additives as oil improvers, E.P. agents, rust inhibitors and sequestering agents. However, none of these methods have been effective for their intended purposes. As a result, a need continues to exist for water-glycol fluids which have good lubricating properties.
There has been proposed a water-glycol based hydraulic fluid comprising a water soluble alkylene oxide adduct of a polyamide. This fluid has improved lubricating properties, but it has poor shear stability.